Microstructural Design of Silicon Nitride with Improved Fracture Toughness: II, Effects of Yttria and Alumina Additives

Significant improvements in the fracture resistance of self-reinforced silicon nitride ceramics have been obtained by tailoring the chemistry of the intergranular amorphous phase. First, the overall microstructure of the material was controlled by incorporation of a fixed amount of elongated ß-Si₃N₄ seeds into the starting powder to regulate the size and fraction of the large reinforcing grains. With controlled microstructures, the interfacial debond strength between the reinforcement and the intergranular glass was optimized by varying the yttria-to-alumina ratio in the sintering additives. It was found that the steady-state fracture toughness value of these silicon nitrides increased with the Y:Al ratio of the oxide additives. The increased toughness was accompanied by a steeply rising R -curve and extensive interfacial debonding between the elongated ß-Si₃N₄ grains and the intergranular glassy phase. Microstructural analyses indicate that the different fracture behavior is related to the Al (and O) content in the ß´-SiAlON growth layer formed on the elongated ß-Si₃N₄ grains during densification. The results imply that the interfacial bond strength is a function of the extent of Al and Si bonding with N and O in the adjoining phases with an abrupt structural/chemical interface achieved by reducing the Al concentration in both the intergranular phase and the ß´-SiAlON growth layer. Analytical modeling revealed that the residual thermal expansion mismatch stress is not a dominant influence on the interfacial fracture behavior when a distinct ß´-SiAlON growth layer forms. It is concluded that the fracture resistance of self-reinforced silicon nitrides can be improved by optimizing the sintering additives employed.     

Modelistic interpretation of the power response of a polymer electrolyte fuel cell

It has been demonstrated recently that the power response of a polymer electrolyte fuel cell can be interpreted on the basis of a quadratic logistic differential equation. As this is a conceptual approach, the power response was obtained in this work from structural models in order to compare the results with the logistic equation. In this way it is possible to evaluate the contribution of the different effects that limit the response of the cell. It was found that the best comparison between the logistic equation and the structural model is obtained when the dependence of the membrane resistance with current density is considered.     

What types of neighbourhoods are redlined?

This paper presents a case study of lending behaviour in the city of Rotterdam, the Netherlands. It shows what types of neighbourhoods are redlined by analyzing redlining data of the two largest suppliers of mortgage loans and comparing it to social-demographic and housing market data at the neighbourhood level. Although this approach cannot explain redlining, it can show which factors are related to lending behaviour. Low income, unemployment and ethnicity are strongly positively correlated to redlining. A discriminant analysis shows that the interaction between low income, unemployment or ethnicity on the one hand, and the average value of sold units on the other hand can best approximate redlining. Lastly, this paper also highlights the importance of scale. Manuel B. Aalbers formerly at: Amsterdam institute for Metropolitan and International Development Studies (AMIDSt), University of Amsterdam, Nieuwe Prinsengracht 130, 1018 VZ Amsterdam, The Netherlands     

Three different phototrophic microbial communities colonizing a single natural shelter containing prehistoric paintings

Three different cases of deterioration were identified in a single natural shelter containing prehistoric paintings. The microbial communities induced by run-off water, a black crust covering a portion of the shelter, and a cryptoendolithic microbial community, were studied. Molecular analyses based both on DNA and RNA were performed to identify the major components of these microbial communities present and metabolically active, respectively, at the studied location. While similar microbial communities were expected due to the proximity of the studied sites, clearly different communities were detected suggesting that specific microorganisms adapt to different micro-environments. Phototrophic microorganisms represented the major portion of total RNA and DNA in the studied microbial communities. Run-off water induced the presence of a large proportion of putative photosynthetic bacteria belonging to Chloroflexi. Different groups of bacteria were associated to phototrophs and were represented by Alpha, Beta, Delta, and Gammaproteobacteria, Acidobacteria, Actinobacteria, Bacteroidetes, and Planctomycetes. The present study confirms a need for careful monitoring of microbial communities associated to distinct cases of deterioration even within a single location, underlining the interest of phototrophic microorganisms as indicators of colonization on cultural heritage at natural shelters.     

Removal of natural organic matter and THM formation potential by ultra- and nanofiltration of surface water

Natural organic matter (NOM) and trihalomethane formation potential (THMFP) removal were evaluated by ultrafiltration (UF) and nanofiltration (NF). Ten different raw water sources in Alicante province (SE Spain) were analysed. Five types of membranes of different materials were tested with a dead-end-type stirred UF cell. Additional measurements, such as dissolved organic carbon, ultraviolet absorbance (254nm), THMFP, ion concentration, pH, conductivity, etc. were made on raw water, permeates and concentrates. The SUVA value was used to determine the hydrophobicity of the water analysed. The elimination of NOM and THMFP is correlated with the molecular weight (MW) of NOM determined by size exclusion chromatography (SEC). The flux decline trends were correlated with cation concentration. NOM removal by UF is low, which correlates with the average MW determined by SEC with an average value of 922g/mol (between 833 and 1031g/mol). However, the NOM removal obtained with the NF90 and NF270 NF membranes for all water sources is almost complete (90%). THMFP removal is related to hydrophobicity and permeability of membrane. The NFT50 membrane removes almost 100% of the THMFP of more hydrophobic waters.     

Effects of indoor drinking water handling on trihalomethanes and haloacetic acids

In this study, different tap water handling strategies were investigated to evaluate the effects on two principal chlorinated DBPs, trihalomethanes (THMs) and haloacetic acids (HAAs). Tap water samples collected in the Quebec City (Canada) distribution system on a spatio-temporal basis were subjected to diverse indoor handling scenarios: storing water in the refrigerator, boiling water followed by storage and, finally, filtering water with a point-of-use commercial pitcher also followed by storage. In the first two cases, the use of covered and uncovered pitchers was investigated separately, while in the last case, both the use of new and used filters was compared. In all cases, maximum storage time was 48h. Results demonstrated that in some cases, water handling scenarios have considerable effect, and in other cases, little or no effect. Removal of THM concentrations by simple storage was high (on average 30%) and very high by boiling and filtering with subsequent storage in the refrigerator (on average, 87% and 92%, respectively). In scenarios where water was stored in uncovered pitchers (with or without previous boiling and filtering), the THM decrease was higher for increased storage times. However, storage did not have any effect on HAAs, whereas boiling decreased levels of trichloroacetic acid (TCAA) (on average 42%) and increased levels of dichloroacetic acid (DCAA) (on average 35%), resulting in unchanged average levels of total HAAs. The use of the filtration pitcher decreased HAA levels dramatically (on average 66%). Percentages of change in chlorinated DBPs in the different scenarios varied according to initial concentrations in tap water (baseline water), that is, according to the spatio-temporal variations of these substances in the distribution system. On the basis of these results, the paper discusses implications regarding public health protection and exposure assessment for epidemiological studies.     

Characterization of the Fouled Ballast Layer in the Substructure of a 19th Century Railway Track Under Renewal

The purpose of these field and lab studies undertaken during rehabilitation work being done on an ancient railway line was to characterize a layer of ballast fouled with soil found in the track substructure. The field studies included the characterization of the thickness, grain size distribution and void ratio of the fouled ballast layer, as well as a large number of plate load tests, both on the fouled ballast layer and on the subgrade. The resilient behaviour of the fouled ballast was evaluated in the lab by cyclic triaxial tests on large size reconstituted specimens with distinct fouling indexes (different grain size distribution) and distinct humidity states (dry or wet). The results obtained were used as support for the decision to maintain the fouled ballast layer under the new sub-ballast in a number of stretches of the renewed line.     

Advanced model for predicting the fire response of concrete filled tubular columns

In this work, a nonlinear finite element three-dimensional model is presented and validated in order to study the behaviour of axially loaded concrete filled tubular (CFT) columns with circular cross-section exposed to fire. A realistic sequentially coupled nonlinear thermal-stress analysis is conducted for a series of columns available in the literature. The model is validated by comparing the simulation results with the real fire resistance tests. By means of this model, and extensive sensitivity analysis is performed over a wide range of aspects concerning the finite element modelling of the problem under study, including new key factors not studied previously. Based on this sensitivity analysis several modelling recommendations are given in this paper, which will be useful for future research work. The validated numerical model is furthermore employed to study and discuss the Eurocode 4 Part 1–2 simple calculation model, which is deeply analysed in this paper.     

A new approach for project control under uncertainty. Going back to the basics

In this paper we propose a new methodology for project control under uncertainty. In particular, we integrate Earned Value Methodology (EVM) with project risk analysis. The methodology helps project managers to know whether the project deviations from planned values are within the “expected” deviations derived from activity planned variability. Although the methodology is new and innovative, we only go back to the fundamentals of project simulation to generate the “universe” of possible projects, according to the assumed variability of project activities. Then we organize and gather the information in order to make the data coherent with EVM. We explain the steps to implement the methodology and we show three case studies. The methodology makes explicit that the schedule and budget resulting from traditional methods like PERT are statistically very optimistic.